On certain regularities of the response of a medium upon generation of the triple frequency of a few-period laser pulse in the case of its passage through an optically thin nonlinear layer

The response of a medium at the triple frequency under the action of few-period laser pulses is considered within the framework of the thin-optical-layer approximation for the case where the triple frequency is close to the natural frequency of the linear oscillator. It is shown that a bistable dependence of the polarization amplitudes on the external action amplitude simultaneously appears at both the natural frequency and the frequency of the external action. This allows one, in particular, to reveal the presence of bistability of the regime of third harmonic generation in a physical experiment by using the transmitted radiation of the acting pulse. A decrease in the duration of the pulse incident on the medium leads to an increase in the hysteresis loops. The effect of the absolute phase of a short pulse on the spectral composition of the response of the medium is studied. For pulses of medium duration, in addition to the resonant response of the medium, the presence of the dynamic response of the medium at the triple frequency was revealed, despite a detuning of the latter from the resonance. The presence of this frequency in the spectrum of the response of the medium results in the dependence of the resonant response of the medium on the absolute phase even for a sufficiently long pulse containing tens oscillations of the electric field strength under the pulse envelope. To obtain the dynamics of the spectral lines from the results of computer simulation, the Fourier-Gabor method is used, the applicability of which is demonstrated by the comparison of the results obtained on its basis with the corresponding analytical dependences.     

Mutual influence of the spectral components of perturbations on the modulation instability frequency interval in a cubic nonlinear medium

The influence of the shape of a Gaussian pulse propagating in a cubic nonlinear medium on the development of its modulation instability is analyzed, taking into account the time dispersion of the nonlinear response as well. Analytical dependences of the frequency interval of the perturbation development on the dispersion of the nonlinear response, the pulse duration, and the number of simultaneously acting perturbations, which affect each other due to the inhomogeneous pulse shape, are obtained. It is shown that, with increasing dispersion of nonlinear response, the frequency interval of the instability development initially narrows and then expands. Qualitative agreement between the analytical dependences and the results of computer simulation is demonstrated. The frequency interval of the modulation instability development in a medium without the nonlinear response dispersion that was previously determined by different authors under the assumption of “long” pulses is found to coincide with that obtained in this study for the Gaussian pulse under the assumption of the simultaneous action of a large number of perturbations.     

Study of nonlinear propagation of chirped optical pulses in laser amplifying medium

In chirped pulse amplification, the amplification of an optical pulse in laser amplifying medium is a very important link. After considering dispersion, Kerr nonlinear effect, gain distribution and loss of the medium, the physical model that the optical pulse propagates in the medium has been established, which is suitable for the general situation where the optical pulse propagates in the amplifying medium. Using a split-step Fourier method, the propagation state of a chirped optical pulse in the amplifying medium has been numerically simulated, and the influences of the gain dispersion of the medium and the frequency detuning of the pulse on the optical pulse have been discussed emphatically. The results show that, with the amplification and propagation of the ultrashort chirped pulse in the amplifying medium, gain saturation and Kerr nonlinear effect of the medium will cause distortion of the optical pulse. For the optical pulse with a wideband spectrum, gain dispersion will cause the gain narrowing effect, so it is equivalent to a loss mechanism. The frequency detuning of the optical pulse will cause distortion of the pulse, which can be used to weaken the impact of gain saturation, thus reshaping the optical pulse.     

Theory of passive mode locking of colour-center lasers by a fast saturable absorber

A theoretical model of passive mode locking of colour-center lasers by an effective fast saturable absorber is presented where a hybrid saturation scheme is applied to consider the interaction of the mode-locked pulse with the gain medium. Intracavity group-velocity dispersion and Kerr-type nonlinearity are taken into account and their combined action can result in solition-like pulse shaping and shortening. Chirped steady-state solutions whose carrier frequency can be shifted with respect to the central frequency of the gain are found in a limited range of the parameters specifying the problem.     

Photoprocesses with the sub-Doppler spectral selectivity excited by the monochromatic optical pulse in a thin gas cell

The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degenerate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of particles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in comparison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub-Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high-selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.     

Temporal characteristics of few-cycle pulse on- and non-resonance propagating in a ladder-type atomic medium

Time evolution characteristic of few-cycle pulse propagating in a ladder-type atomic medium is investigated. It is shown that, time evolution of few-cycle pulse has significant difference in the both cases on- and non-resonance. In the non-resonant case, if the pulse central frequency is twice as large as medium atomic transition frequency, the pulse form (including carry-envelope phase, pulse duration, oscillation amplitude and frequency) remains unchanging or remains unchanging basically (only carry-envelope phase has some variation) in the propagation process, which means that self-induced transparency (SIT) or approximate transparency phenomenon appears. However, in the resonant case, the pulse form changes obviously in the propagation process. In addition, there is evident difference in the propagating velocity for on- and non-resonance when the pulse area is smaller.     

Room-temperature Tm, Ho:LuLiF(4) laser with a novel quasi-end-pumping technique

A novel diode-pumped, efficient, quasi-end-pumping scheme that uses two lens ducts to achieve homogeneous pumping in a gain medium was proposed and evaluated. With this technique, room-temperature laser action at a pulse repetition frequency up to 20 Hz was demonstrated in 5%Tm, 0.5%Ho:LuLiF(4), which we grew by the Czochralski method. At 10 Hz a long-pulse laser output energy in excess of 24 mJ and a slope efficiency with respect to the incident pump energy of 10.7% were obtained.     

Generation and propagation of undamped temperature waves under pulse action on a target surface

The physical mechanism of undamped temperature waves and the conditions of generation thereof in media with a finite time of local thermal relaxation are examined. The wave frequency depends on the relaxation time and the parameters of the pulse action on a medium, and the lowest frequencies correspond, in particular, to air. These effects can be used to identify and investigate this action. It is demonstrated that the character of wave reflection from the interface between different media depends on the state and the finish quality of the given surface. In particular, in the ideal case of a sharp interface between air and metal, temperature waves are totally reflected at the interface. On the contrary, if there is a “blurred” interface, reflections can be almost completely suppressed and waves not absorbed in air penetrate into another medium and are rapidly attenuated in it. Other combinations of media can also lead to an analogous phenomenon, which makes it possible to develop remote heating systems by which distant environments are warmed without thermal energy losses in the intermediate medium.     

Effect of switching a magnetoplasma medium on the duration of a monochromatic pulse

The main effect of switching a magnetoplasma medium is to split the source wave into new waves whose frequencies are different from the source wave. In addition, if the source is a monochromatic pulse, the duration of the pulse is altered. Analytical expressions for the pulse duration of the various characteristic waves in a magnetoplasma are derived. The variations of the pulse duration with the source frequency, the plasma frequency and the cyclotron frequency are illustrated. The principle of the change in the pulse duration may be used to diagnose dynamically the time — varying parameters of a magnetoplasma medium.     

The influence of the shape and chirping of a femtosecond pulse on the self-formation of solitons propagating in a cubic nonlinear waveguide

The influence of the duration, initial form, and chirping of a femtosecond pulse on the possibility of formation of solitons in a cubic medium, which, in addition, introduces a homogeneous phase shift, is considered. The time dispersion of the nonlinear response of the medium is taken into account in this consideration. It is shown that the formation of solitons results not only from the linear phase modulation of the input pulse but also from the modulation of frequency, which varies the phase modulation along the pulse in only one direction (it either increases or decreases the frequency). The analytical soliton solution constructed confirms the results of computer simulation.     

Generation of optical soliton pulses smoothly tunable in a wide frequency range in silica fibers with variable dispersion

The possibility of the efficient conversion of the frequency of an optical soliton pulse in fibers with length-variable dispersion is physically analyzed and experimentally investigated. The effect is based on the Raman transformation of the frequency of an ultrashort pulse, which can be varied with high efficiency due to the compression mechanism of maintaining a high intensity of the pulse in a medium with monotonically decreasing anomalous dispersion. A smooth rearrangement of the spectrum of the soliton pulse 90 fs in duration in the range 1.58–1.75 μm is demonstrated.     

Transformation of a Gaussian pulse reflecting from a nonlinear medium

The transformation of the envelope of a Gaussian pulse incident on and reflecting from a nonlinear dispersive medium is studied. It is shown that the pulse envelope transforms considerably upon reflection near the optical resonance frequency and the nonlinearity of the medium may appreciably distort the reflected pulse. Away from the resonance frequency, conditions may arise when the shift of the reflected pulse does not lead to the loss of the Gaussian form. In this case, the influence of nonlinearity is insignificant.     

Screw-type transparency in a three-level medium

A solution is obtained for the system of Maxwell-Schrödinger equations describing the propagation of a laser pulse through a three-level Λ medium in which the initial atomic populations are in a specially prepared coherent superposition of low-energy states. This solution describes a new type of transparency which is characterized by the presence of conversion of the frequency of the incident pulse and can be used, specifically, to produce “optical keys” and frequency converters for laser radiation.     

Rabi frequency and optical Bloch equations for a short femtosecond optical pulse

We investigate the interaction of a multilevel atom with a short femtosecond optical pulse beyond the scope of the rotating-wave approximation. We consider the behavior of a three-level atom under the action of two optical pulses resonant with two allowed transitions and a two-level atom under the action of a resonant optical pulse. It is shown that the imaginary part of the off-diagonal elements of the density matrix oscillates with high frequency due to the counter-rotating terms. It is shown also that an effective time-dependent addition to the Rabi frequency exists, which is of importance for ultrashort optical pulses.     

粒子数密度对飞秒Gauss型脉冲传播及光谱特性的影响

利用预估校正-时域有限差分(PC-FDTD)法求解全波Maxwell-Bloch方程,研究介质粒子数密度(N)对飞秒Gauss型激光脉冲在Λ型三能级原子介质中传播及光谱特性的影响.结果表明:小面积2π脉冲在不同N介质中都不发生分裂,脉冲频谱基本没有新的高频成分产生,随N增大中心频率附近光谱强度明显减小.面积4π脉冲,在N较大的稀疏介质及稠密介质中都产生分裂,在稀疏介质中随N增大频谱展宽幅度及高频成分强度增大,但在稠密介质中频谱展宽变小且远小于N较大时的稀疏介质情况.大面积8π脉冲,脉冲分裂情况与4π脉冲情况相似,但随N增大频谱展宽幅度及高频成分强度单调增大,且在稠密介质中的频谱展宽幅度及高频成分强度远大于N较小的稀疏介质情况.     

A computational theory of short pulse generation

The semi-classical equations for the electric field and population inversion in a two-level medium are simplified for the case of a saturable absorber cell which is shorter than the length of fluctuations in the optical-pulse envelope. The spatial coordinates in the cell are effectively lumped, and the integration is reduced to the one dimension of time. The pulse is passed through an amplifying medium and frequency selective elements by a fast Fourier transform into its spectral components. The case of pulse formation in a ruby laser, starting from a noise fluctuation is computed for the parameters of a recently reported accurate measurement of pulse duration. Good agreement is obtained with experiment. Finally, a pulse length is predicted for a cavity containing an etalon which cancels gain narrowing.     

Optical bistability at two-wave action of light on a molecular gas

An optically bistable two-wave scheme implemented upon absorption of light energy by a molecular gas is proposed. Switching from one state into another is performed by a pulse at one of the two frequency. At the same time, the medium is continuously exposed to light of another frequency. It is shown that, under certain conditions, the system switched to the upper state remains in this state for an arbitrarily long time. This effect makes it possible to implement long-term dynamic memory and logical operations. A two-wave action on a molecular gas also decreases several tens or hundreds of times the intensities necessary for switching a system from one state to another.     

Generation of picosecond optical pulses from single heterostructure GaAs diode laser and their emission characteristics

Emission characteristics of a single heterostructure GaAs diode laser are reported using a simple driver circuit. It provides a single picosecond time duration optical pulse, a pulse train or a broad optical pulse depending on the amplitude and time duration of the electrical pump pulse. Results show that relaxation oscillation frequency depends on the amplitude of pumping current pulse as well as on some inherent property of diode laser, which seems to be the level of impurity in lasing medium. Variation of relaxation oscillation frequency with amplitude of current pulse shows only the qualitative agreement with the reported theoretical predictions.